IMDs are used to treat patients suffering from a variety of conditions. IMDs can be utilized in a variety of applications such as therapy delivery and monitoring physiological parameters in patients. Some IMDs are designed to generate and deliver electrical pulses to stimulate body tissue, muscles, nerves, brain cells, body fluid, or the like. Uses for IMDs can be found in many different areas of medicine such as cardiology, endocrinology, hematology, neurology, muscular disorders, gastroenterology, urology, ophthalmology, otolaryngology, orthopedics, and similar medical subspecialties.
Examples of IMDs used in cardiac applications are implantable pacemakers and implantable cardioverter-defibrillators. Such electronic medical devices generally monitor the electrical activity of the heart and provide electrical stimulation to one or more of the heart chambers when necessary. For example, pacemakers are designed to sense arrhythmias, i.e., disturbances in heart rhythm, and, in turn, provide appropriate electrical stimulation pulses at a controlled rate to selected chambers of the heart in order to correct the arrhythmias and restore the proper heart rhythm. The types of arrhythmias that may be detected and corrected by such IMDs include bradycardias (unusually slow heart rates) and certain tachycardias (unusually fast heart rates). Some IMDs monitor the electrical activity of the heart, but do not provide electrical stimulation.
Other IMDS may be used to monitor one or more hemodynamic characteristics of a patient, such as the pressure of blood within a particular vessel or chamber of the heart, the oxygen saturation level of blood of the patient (e.g., arterial blood), the volume of blood supplying a particular tissue site, and the like. Example medical devices that monitor hemodynamic characteristics of a patient include pressure sensors, pulse oximeters, blood flow sensors, hematocrit sensors, and tissue perfusion sensors. The pressure indicated by an implanted pressure sensor may be compared to an external pressure to determine gauge pressure of a body fluid, e.g., blood. One type of pressure sensor is a capacitive pressure sensor. One type of pulse oximeter includes at least one light source that emits light through a portion of blood-perfused tissue of a patient, and an optical detector that senses the emitted light that passed through the blood-perfused tissue. The time-varying optical amplitude measured by the optical detector can be processed, using algorithms known by those skilled in the art, to produce a measure of arterial blood oxygen saturation of the patient.